Same rendering gives different noise amount
I'm not sure i have an explanation for that.

The faulty frames were rendered by two different users (5 for one user, 1 for the other), on different hardware (4 cpu, 2gpu). Only "windows" was the same but i don't know which version.

The Blender version used by SheepIt is actually the one from Blender Foundation, I just remove the blenderplayer.

Here are the binaries to verify, if that helps https://www.sheepit-renderfarm.com/media/binaries/

I'm also not modifying any seed value. Here is the python script I uss on a CPU render:

import bpy
bpy.context.user_preferences.system.compute_device_type = "NONE";
bpy.context.scene.cycles.device = "CPU";
bpy.context.user_preferences.system.compute_device =  "CPU";
bpy.context.scene.render.tile_x = 32;
bpy.context.scene.render.tile_y = 32